With the advancement of technology and the need for instantaneous information, the ability to transfer digital information from one location to another, such as from a central office (CO) to a customer premises (CP), has become more and more important. Allowing for increasing data transmission rates has become a requirement, as opposed to an option.
As part of the system responsible for proper transmission and reception of data in a broadband network, one end-point within the network may comprise the following. An analog front end that (AFE) interfaces between an analog transmission medium, such as an analog telephone line, and digital processing circuits. The AFE, among other functions, converts a digital signal into a continuous time analog signal. Particular to the case of a digital subscriber line (DSL) AFE with an integrated line driver, the AFE also puts this converted signal on a two-wire pair, or twisted pair. The AFE circuit performs this function by using the combination of a digital-to-analog converter and an analog-to-digital converter. The digital-to-analog converter receives digital signals from a digital signal processor (DSP), and converts them to analog signals, upon which the analog signals are transmitted to a line driver. The analog-to-digital converter receives analog signals, converts them to digital signals, and furnishes them to the DSP. The AFE may also incorporate other elaborate analog signal processing. Once the analog signal is transmitted to the line driver, the line driver drives the analog signal through the two-wire pair in accordance with a required power particular to the application considered and the type of line driven.
The distance for transferring information over a twisted pair is typically large. As an example, the distance from a residence to a central office may be in excess of 15,000 feet. Unfortunately, twisted pairs attenuate signals in proportion to both line length and frequency. As the line length and frequency increases, the performance of specific DSL transmission, such as, but not limited to, asynchronous DSL (ADSL), decreases. Therefore, there is a need for preventing attenuation when transferring digital information over long distances. In addition, line drivers usually do not provide amplification of the signals to be transmitted, resulting in line drivers not being a potential vehicle used for minimizing attenuation.
To assist in addressing the above mentioned information transfer difficulty, repeaters have been introduced into networking systems. Repeaters provide a means for expanding the distance for transferring information over a twisted pair, without allowing attenuation. Typically, repeaters demodulate and regenerate a signal received from a first end point (CO) and send the signal out to a second endpoint (CP) on a line such as a twisted pair. This process allows received information to be transmitted over longer distances without attenuation.
Due to repeaters typically being located distant from any power source, power is provided to the repeaters via the twisted pair, which is otherwise referred to as line powering. Specifically, a DC current is transmitted to the repeater via the twisted pair. The power may also be transmitted to additional repeaters that are connected via a twisted pair. Typically, a series of inductors are located within a repeater to provide a DC path such that all repeaters receive necessary power to perform demodulation and regeneration.
Unfortunately, typical repeaters suffer from direct coupling of signals from one side of the repeater to the other side as explained herein below. Ordinarily, repeaters comprise a first and second DSP, wherein signals are received by the first DSP and demodulated, after which the demodulated signal is transmitted to the second DSP that re-modulates the demodulated signal. The re-modulated signal is then transmitted to a destination endpoint or another repeater via the twisted pair. A power feed, or DC power path, provides the path through which noise is coupled from one DSP to the next DSP. This coupling is otherwise referred to as crosstalk. The effect of the crosstalk results in poor repeater performance due to the inability of the receiving part of the repeater to effectively cancel uncorrelated input signals. The level of crosstalk can be somewhat reduced by the inductors located within the repeater, which provide a single pole low pass filter whose cutoff frequency is inversely proportional to the value of the inductor. Hence, the larger the inductor, the lower the cutoff frequency. In a repeater arrangement where space is limited, large inductors are not practical, nor desirable.